1. Field of the Invention
The present invention relates to a display device capable of displaying an image toward each of a plurality of viewpoints, and to a terminal device, and particularly relates to a display device capable of reducing a deterioration in display quality due to the structure of the display device, to a terminal device, and to a display panel and optical member that can be suitably used in the display device and terminal device.
2. Description of the Related Art
Due to recent technological development, display panels are deployed and used in various locations in a range of devices that includes monitors, television receivers, and other large terminal devices; notebook personal computers, cash dispensers, vending machines, and other mid-sized terminal devices; and personal TVs, PDAs (Personal Digital Assistance: personal information terminal), mobile phones, mobile gaming devices, and other small terminal devices. Because of their thin profile, light weight, small size, low energy consumption, and other advantages, display devices that use liquid crystals in particular are deployed in a large number of terminal devices. Current display devices show the same content as is visible in the frontal direction even when observed from a viewpoint that is not in the frontal direction, but display devices that enable a different image to be viewed according to the viewpoint are being developed, and are anticipated as next-generation display devices. A three-dimensional image display device can be cited as an example of a device that is capable of displaying a different image to each of a plurality of viewpoints. A three-dimensional image display device must have a function for providing different images for left and right viewpoints, i.e., parallax images to the left and right eyes.
Numerous three-dimensional image display systems have been investigated in the past as methods for specifically implementing the function described above, and these systems can be broadly classified as systems that use eyeglasses and systems that do not use eyeglasses. Systems that use eyeglasses include anaglyph systems that use a color difference, polarized-light eyeglass systems that use polarized light, and other systems, but the inconvenience of eyeglasses is inherent to these systems. Eyeglass-free systems that do not use eyeglasses have therefore been heavily investigated in recent years.
Eyeglass-free systems include lenticular lens systems, parallax barrier systems, and the like. As described in Japanese Laid-open Patent Application No. 2004-280079, a lenticular lens system uses a lenticular lens as means for dividing an image with respect to a plurality of viewpoints. In the lenticular lens, one of the surfaces is composed of a flat surface, and the opposite surface has a plurality of semi-cylindrical convex parts (cylindrical lenses) extending in one direction that are formed so as to be parallel to each other in the longitudinal direction thereof. In a lenticular lens three-dimensional image display device, the lenticular lens and the display panel are arranged in sequence from the direction of the observer, and the pixels of the display panel are positioned at the focal plane of the lenticular lens. In the display panel, pixels for displaying a right-eye image and pixels for displaying a left-eye image are arranged in alternating fashion. At this time, groups of adjacent pixels correspond to the convex parts of the lenticular lens. The light from the pixels is thereby allocated to the directions of the left and right eyes by the convex parts of the lenticular lens. Different images can be recognized by the left and right eyes, and the observer can recognize a three-dimensional image.
A parallax barrier system uses a barrier (light-blocking panel) in which numerous openings in the shape of narrow vertical stripes, i.e., slits, are formed as a means for dividing an image. Groups comprising pixels for displaying a left-eye image and pixels for displaying a right-eye image are arranged to correspond to the slits of the parallax barrier. As a result, the pixels for displaying the left-eye image are blocked by the barrier and unable to be seen by the observer's right eye, and only the pixels for displaying the right-eye image are visible to the observer's right eye. In the same manner, the pixels for displaying the right-eye image are blocked by the barrier and unable to be seen by the observer's left eye, and only the pixels for displaying the left-eye image are visible to the observer's left eye. As a result, the observer can recognize a three-dimensional image when a parallax image is displayed.
When the parallax barrier system was first proposed, the parallax barrier was positioned between the pixels and the eyes, thereby obstructing the view and creating the problem of poor visibility. However, recent developments in liquid crystal display devices have enabled the parallax barrier to be placed behind the display panel, thereby improving visibility. Parallax barrier three-dimensional image display devices are therefore being actively investigated. However, a parallax barrier system is a system for “hiding” unnecessary light rays using a barrier, whereas a lenticular lens system changes the propagation direction of light, and the lenticular lens system has an advantage in that there is no reduction of the brightness of the display image in principle. A lenticular lens system is therefore being investigated for application in mobile devices and the like in particular, in which high-luminance display and low power consumption are important.
A multi-image simultaneous display device capable of displaying a plurality of different images to multiple viewpoints at the same time has been developed (see Japanese Laid-open Patent Application No. 06-332354, for example) as another example of a device that is capable of displaying different images to a plurality of viewpoints. In this display, a function for allocating an image using a lenticular lens is utilized to display a different image for each observation direction simultaneously under the same conditions. A single multi-image simultaneous display device can thereby simultaneously provide images that differ from each other to a plurality of observers who are positioned in different directions from each other with respect to the display device. According to Japanese Laid-open Patent Application No. 06-332354, the use of this multi-image simultaneous display device enables reductions in installation space and electricity cost in comparison to preparing a number of normal single-image display devices equal to the number of images that are to be displayed at one time.
Since a different image can thus be displayed to each different viewpoint, display devices in which a lenticular lens, parallax barrier, or other optical member is provided are being actively investigated. However, the present inventors have discovered that several problems occur when an optical member is merely provided, and the inventors have proposed means for overcoming these problems.
For example, when a semi-transmissive display panel and a reflective display panel that is provided with a reflecting panel having an concavo-convex (i.e. meaning “protruding” here and hereafter) structure in the pixels is used as described in Japanese Laid-open Patent Application No. 2004-280079, a region occurs in which the luminance of the display is partially reduced according to the observation position, the display appears to darken in the position in which the luminance was reduced when the observation position is changed, and in some cases, a dark lined pattern is observed to be superimposed on the image. The display quality is visibly reduced by this change in the luminance of the display. The cause of this problem is that the reflection angle varies according to the tilt angle of the tilted surfaces that constitute the concavo-convex structure when external light that is focused by the lenticular lens is reflected by the concavo-convex structure formed on the reflecting panel. Japanese Laid-open Patent Application No. 2004-280079 therefore proposes a method for providing the lenticular lens so that the focal distance thereof differs from the distance between the reflecting panel and the lens; a method for setting the tilted surfaces of the concavo-convex structure so that the concavo-convex structure reflects the light multiple times that is focused by the lenticular lens; and a method for setting the concavo-convex structure so that the probability of a tilted surface having a certain tilt angle being present in the concavo-convex structure is uniform among the pixels in the alignment direction of the cylindrical lenses.
As described in Japanese Laid-open Patent Application No. 2006-17820, another problem that occurs when an optical member is provided is that a fringe pattern is superimposed on the display image, and display quality is severely reduced by the effect of the concavo-convex structure formed in the illumination member of the transmissive display device when an optical member used for image separation is incorporated into a transmissive display device. The cause of this problem is that an in-plane distribution occurs in the directivity of light rays emitted from the illumination member due to the concavo-convex structure that is formed in the illumination member, and this in-plane distribution is visibly magnified by the optical member for image separation. This problem is exacerbated as the concavo-convex structure of the illumination member is brought closer to the focal plane of the lenticular lens by profile reduction and the like. Japanese Laid-open Patent Application No. 2006-17820 therefore proposes a method for minimizing the reduction of display quality by making the distance between adjacent convex parts in the concavo-convex structure smaller than the value obtained by multiplying the distance between the concavo-convex structure and the pixels by the lens pitch of the lenticular lens and dividing the result by the focal distance, and by varying the concavo-convex structure according to the lenticular lens used.
However, such problems as the following are apparent in the methods described above for overcoming the issues described above, i.e., the reduction in display quality caused by the concavo-convex structure formed in the reflecting panel, or the image quality reduction due to the concavo-convex structure formed in the illumination member, by varying the performance of the concavo-convex structure or the lenticular lens. Specifically, problems occur in that the lens or other optical element, as well as the concavo-convex structure of the reflecting panel, and the concavo-convex structure of the illumination member must be changed. There may be no options for modification particularly when a common, standardized product is used for the members described above. In the case of a lens, illumination member, or other member that has a three-dimensional shape, a change may have to be made from the molding stage in the case of a surface shape variation, which may involve a large-scale modification. There is therefore a need for a method for overcoming the aforementioned problems by a simpler means without modifying the lens surface or the concavo-convex structure.
As a result of concentrated investigation of display devices provided with a lenticular lens, a parallax barrier, or other optical member, the inventors discovered that the pattern of boundary regions between adjacent pixels or other regions that do not contribute to the display in these display devices is observed as parallel lines in the arrangement direction of the lenses or slits, and that the problem of reduced image quality occurs.